Anthony Ferrara has started a new series of posts to his blog that will introduce you to the source behind the project - the code that makes PHP tick. In this first part of the series, he gives more of an overview of things like where you can find the source and some basic C concepts.

In this post, we'll walk through the basics of the PHP application: where to find it, the general structure of the codebase and a few really fundamental concepts about the C language. To be clear, the goal of the series is to get a reading comprehension of the source code. [...] Additionally, this series is going to be based off the 5.4 codebase. The concepts should be pretty much the same from version to version, but this way there's a defined version that we're working against (to make it easier to follow later, when new versions come out).

He includes the link to the PHP SVN repository to grab the latest source, talks some about the general structure of the codebase and basic C information in case you're not versed in the language (like variables and pre-processor instructions).